Method of making nitride and oxide electrodes on a solid electrolyte

ABSTRACT

A method is disclosed for bonding nitride and oxide film electrodes on ceramic substrates, which can be used in thermo electric generators. Hydride powders in suspension are coated on a substrate, and heated to selected temperatures for selected periods of times in alternating atmospheres of hydrogen and nitrogen, or oxygen. The method produces satisfactory film electrodes, including electrodes which are greater than 10 microns in thickness.

This invention was made with Government support as a result of asubcontract from CERAMATEC under Prime Contract No. N00014-87-C-0858awarded by the Office of Naval Research. The Government has certainrights in this invention.

This is a division of application Ser. No. 07/438,067, filed Nov. 20,1989, now U.S. Pat. No. 5,114,743.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method for forming a nitride or oxide filmelectrode on a solid electrolyte, and more specifically to a method forforming a nitride or oxide film electrode on beta"-alumina for use in athermo-electric generator.

2. Description of the Related Art

Thermo-electric generators convert heat energy from a heat source toelectricity. Examples of thermo-electric generators are the sodium heatengine (SHE), alkali metal thermo-electric converter (AMTEC), or liquidmetal thermo electric converter (LMTEC), which generate electricity byexpanding a metal, for example sodium, across a solid electrolyte. Insuch generators, the solid electrolyte separates a closed container intoa first and second reaction zone. The first reaction zone containsliquid sodium, while the second reaction zone contains an electrode incontact with the solid electrolyte. As the first reaction zone isheated, the liquid sodium gives up electrons, causing sodium ions tomigrate through the electrolyte to the second reaction zone, to beneutralized at the electrode-electrolyte interface. Neutralized sodiummetal atoms pass through the electrode, then evaporate, pass through acondenser, and are returned to the first reaction zone.

The efficiency of the electrode in the thermo-electric generatordescribed above has been optimized by use of permeable materials whichdo not impede migration of the sodium atoms through the electrode, whilesimultaneously conducting electrons to the solid electrolyte interfaceto neutralize the sodium ions migrating through the electrolyte. A goodexample of a suitable electrode displaying the above features is aporous metal film, such as a nitride, deposited on a ceramicelectrolyte.

Various methods have been proposed for producing an electrode such asthe one described above. Such methods include reactive sputtering, ionplating, electron beam evaporation, and the like. Such methods, however,have a number of drawbacks when used to produce a metal film electrodeon a ceramic substrate for use in a thermo-electric generator.

For example, the conventional methods frequently degrade the mechanicalor ion-conducting properties of the ceramic substrate.

Further, the conventional methods produce a film which does not adherewell to the ceramic substrate over the life of the electrode.

Further, the conventional methods are relatively expensive and extremelytime consuming, producing a serious commercial disadvantage.

Further, the conventional methods produce an extremely dense film on theceramic substrate. It would be advantageous for the metal film to be asthick as possible, since such an electrode would have a low sheetresistance to electron flow. However, due to the high density of thefilm produced by the conventional methods, increasing thickness resultsin high impedance to diffusion of neutralized sodium atoms. Thus, usingthe conventional methods, only a thin electrode can be produced whichhas any reasonable commercial use.

The present invention improves on the conventional methods by providinga method for bonding a film electrode on a solid electrolyte that doesnot degrade the mechanical or ion-conducting properties of theelectrolyte, adheres well to the electrolyte, is quick and inexpensive,and produces thicker films, thus improving conductivity without impedingthe diffusion of sodium atoms.

Additional advantages of the invention will be set forth in thedescription which follows and in part will be obvious from thedescription, or may be learned from practice of the invention. Theadvantages of the invention may be realized and obtained by means of theinstrumentalities and combinations particularly pointed out in theappended claims.

SUMMARY OF THE INVENTION

In accordance with the purposes of the invention as embodied and broadlydescribed herein, there is provided a method for forming a nitride filmelectrode on a surface of a solid electrolyte substrate comprising thesteps of suspending a metallic powder in a liquid having a binderdissolved therein, forming a coating of the liquid suspension on thesurface of the electrolyte, the liquid and binder being evaporablewithout substantial carbonization as it is heated past a first elevatedtemperature, heating the coating in a hydrogen atmosphere to a selectedsecond elevated temperature higher than the first elevated temperatureto convert the coating to a bonded film and increasing the bondabilityof the film to the electrolyte without the electrolyte losingion-conducting properties, subjecting the film in the hydrogenatmosphere to the selected second temperature for a first time period toeffect the required degree of the bonding of the film to theelectrolyte, and subjecting the film at the expiration of the first timeperiod to a nitrogen atmosphere for a second time period effective toconvert the bonded film to a nitride film.

There is further broadly provided a method for forming an oxide filmelectrode on a surface of a solid electrolyte substrate comprising thesteps of suspending a metallic powder in a liquid having a binderdissolved therein, forming a coating of the liquid suspension on thesurface of the electrolyte, the liquid and binder being evaporablewithout substantial carbonization at a first elevated temperature,heating the coating in a hydrogen atmosphere to a selected secondelevated temperature higher than the first elevated temperature toconvert the coating to a bonded film and increasing the bondability ofthe bonded film to the electrolyte without the electrolyte losing ionconducting properties, subjecting the film in the hydrogen atmosphere tothe selected second temperature for a first time period to effect therequired degree of bonding of the film to the electrolyte, andsubjecting the film at the expiration of the first time period to anoxygen atmosphere for a second time period effective to convert thebonded film to an oxide film.

It is preferable that the metallic powder used in the suspending stepsis a metal hydride, and that the metal hydride is selected from a groupconsisting of titanium hydride (TiH₂), zirconium hydride (ZrH₂), hafniumhydride (HfH₂), and vanadium hydride (VH).

It is further preferable that the first elevated temperature is in arange from 100° C. to 400° C.

It is further preferable that the second elevated temperature is in arange from 800° C. to 1100° C., and ideally 900° C.

There is further broadly provided a nitride film electrode for use in athermo-electric generator, formed by the methods described above, havinga low sheet resistance to electron flow and a high permeability foralkali metal atom diffusion, having a thickness of greater than 10microns.

There is further broadly provided an oxide film electrode for use in athermo-electric generator, formed by the methods described above, havinga low sheet resistance to electron flow and a high permeability foralkali metal atom diffusion, having a thickness of greater than 10microns.

DESCRIPTION OF THE DRAWINGS

The accompanying drawings which are incorporated in and constitute apart of the specification, illustrate a preferred embodiment of theinvention, together with the general description given above and adetailed description of the preferred embodiment given below, serve toexplain the principles of the invention.

FIG. 1 is a graph showing voltage-current curves for a thermo-electricgenerator cell using a titanium nitride electrode formed by the methodof the present invention.

FIG. 2 is a graph showing maximum power versus temperature for titaniumnitride electrode formed by the methods of the present invention.

FIG. 3 is a graph showing voltage current curves and maximum powerversus temperature for a thermo-electric generator cell using a titaniumnitride electrode having a film thickness of 28 microns.

FIG. 4 is a graph showing voltage-current curves and maximum powerversus temperature for a thermo-electric generator cell using a zirconianitride electrode having a film thickness of 11 microns.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to the present preferredembodiments of the invention.

A method for forming a nitride film electrode on a surface of a solidelectrolyte substrate for use in a thermo electric generator includes astep of suspending a metallic powder in a liquid having a binderdissolved therein. As broadly described herein, the metallic powder usedin the suspending step is a metallic hydride and may be selected, forexample, from a group consisting of titanium hydride (TiH₂), zirconiumhydride (ZrH₂), hafnium hydride (HfH₂), and vanadium hydride (VH), allof which are readily available as fine powders and decompose at moderatetemperatures. Metal powders may also be used but are generally lesspreferable because it is difficult to grind metals into suitably finepowders. The metallic hydride is suspended in, for example, toluene anddissolved therein with a small amount of a binder such as amethacrylate. A typical formula may be 10 grams of TiH₂ of 1-3 microngrain size, suspended in 250 milliliters of toluene, in which isdissolved 0.5 grams of ELVACITE 2046, (manufactured by DuPont).

In accordance with the invention, a coating of the liquid suspension isformed on a surface of an electrolyte. Preferably, the suspension issprayed on a surface of the substrate which may be, for example, aceramic such as beta"-alumina, in order to uniformly deposit a coatingof the suspension thereon. Other acceptable techniques include brushingor rolling the suspension onto the substrate, screen printing, dippingthe substrate in the suspension, or other methods well-known in the art.

In accordance with the invention, the liquid and binder evaporatewithout substantial carbonization as the electrolyte and suspension areheated past a first elevated temperature. As broadly described herein,the substrate with the applied coating is placed in a conventionalheating chamber, and heated at a constant rate. The binder is removedfrom suspension and evaporated without carbonization as the coatedelectrolyte is heated past a first temperature, being in a range from100° C. to 400° C.

In accordance with the invention, the coated electrolyte is heated in ahydrogen atmosphere to a selected second elevated temperature higherthan the first elevated temperature to convert the coating to a bondedfilm, increasing the bondability of the film to the electrolyte withoutthe electrolyte losing ion conducting properties. The film is subjectedto the second temperature for a first time period to effect the requireddegree of bonding. As broadly described herein, a hydrogen atmosphere isintroduced into the chamber, preferably by a step of flowing a stream ofdry hydrogen over the coated electrolyte, promoting a decomposition ofthe metal hydride and promoting bonding of the metal to the ceramicsubstrate. This bonding step in the hydrogen atmosphere is conducted ata selected second temperature for a first time period. The ideal secondtemperature has been found to be in a range of 800° C. to 1100° C., andideally 900° C., with this temperature maintained for at least 30minutes.

In accordance with the invention, at the expiration of the first timeperiod the film is subjected to a nitrogen atmosphere for a second timeperiod effective to convert the bonded film to a nitride film. Asembodied herein, the flow of gas over the metal film is switched fromhydrogen to nitrogen or a mixture of nitrogen and hydrogen. Preferably,nitrogen is flowed over the film for at least 30 minutes in order tocomplete the nitriding.

To obtain pure nitride electrodes, a step of excluding oxygenimpurities, which tend to displace the nitrogen to create oxides in thefilm, may be included in the process. The step of excluding oxygenimpurities may include, for example, using pure nitrogen rather thanconventional tank nitrogen, or purging the nitrogen lines with hydrogenor inert gas prior to flowing nitrogen. Another method is introducing agetter or a purifier apparatus, comprising a screen coated with thehydride suspension, in the nitrogen lines upstream of the chamber.Oxygen impurities in the nitrogen are scavenged by reacting with thesuspension on the purifier apparatus, leaving pure nitrogen to flow tothe heating chamber.

To obtain pure nitride electrodes, another additional step may beperformed of cooling the nitride film slowly in dry hydrogen. This stepis preferable if impure tank nitrogen has been used in the step offlowing nitrogen and no purification has been performed.

In accordance with the present invention, the method described above maybe also be used to form an oxide film electrode on a surface of a solidelectrolyte substrate for use in a thermo-electric generator, comprisingthe steps of suspending a metallic powder in a liquid having a binderdissolved therein, forming a coating of the liquid suspension on thesurface of the electrolyte the liquid and binder being evaporablewithout substantial carbonization at a first elevated temperature,heating the coating in a hydrogen atmosphere to a selected secondelevated temperature higher than the first elevated temperature toconvert the coating to a bonded film and increasing the bondability ofthe bonded film to the electrolyte without the electrolyte losingionconducting properties, subjecting the film in the hydrogen atmosphereto the selected second temperature for a first time period to effect therequired degree of bonding of the film in the electrolyte, andsubjecting the film at the expiration of the first time period to anoxygen atmosphere for a second time period effective to convert thebonded film to an oxide film.

As embodied herein, the method for forming the oxide film electrode issimilar to that for forming a nitride electrode, with the exception thatan atmosphere of oxygen, or nitrogen with oxygen impurities, is flowedover the metal film in place of impurity-free nitrogen. The oxidesformed may be, for example, TiO, TiO₂, Ti₄ O₇, or others, dependent onthe metal selected and the partial pressure of oxygen used. Thetemperature and times required for the bonding steps are the same asthose disclosed for nitride films and therefore shall not be repeated.

The methods described above have been found to produce films whichadhere well to the ceramic electrolyte. Of particular importance inproducing this adherent bond is the step of heating the film whileflowing hydrogen to preferably 900° C. for least 30 minutes prior toflowing nitrogen.

Films formed using the disclosed methods have an advantage in that theyare less dense than those deposited using conventional sputtering andvacuum deposition methods. Thus, as broadly described herein, nitrideand oxide film electrodes can be formed having a wide range ofthicknesses, including a thickness of greater than 10 microns, allowingfor improved flow of electrons, while providing adequate diffusion ofalkali metal atoms when used in the conventional thermal-electricgenerator. Nitride and oxide film electrodes of up to 28 microns havebeen tested with satisfactory results.

Electrodes formed by the methods of the present invention have producedthe following experimental results.

A titanium nitride coating approximately 6 to 10 microns thick wasapplied to a beta"-alumina tube. A current collector was applied to thecoated tube as follows. A 3 millimeter pitch helical wrap of 0.25millimeter molybdenum wire was placed on the coated tube, and overlayedwith 4 uniformly spaced 3 millimeter flat braided copper conductorsparallel to the tube axis. A second wrap of the molybdenum wire kept thecopper cinched down on the first winding. The tube was placed in asodium heat engine (SHE) cell. The cell was filled from a pressurizedtank of purified sodium at 120° C. Cell temperature was raised abruptlyto 550° C. Voltage-current traces at temperatures up to 775° C. areshown in FIG. 1. Maximum output power, both the observed and thatcorrected to zero internal lead resistance, are shown in FIG. 2. One ofthe cells tested accumulated 2900 ampere hours at 700° C. with noperformance loss. Another cell accumulated 9500 ampere hours attemperatures between 700° C. and 750° C. with no performance loss.

In a second experiment, a beta"-alumina tube having a 25.5 mm outerdiameter, and a 23.2 mm inner diameter was uniformly coated along alength of 16.5 cm on its exterior with titanium nitride, having totalmass of 0.72 grams. If the coating were 100% dense, this wouldcorrespond to a thickness of 10 microns. Microscopic measurements showedthe actual thickness to be 28 microns and therefore, the coating was 64%porous. The coated tube was provided with a current collector asdescribed above and tested in a SHE cell. The voltage-current curves aswell as maximum power versus temperature, obtained by loading the cellat various temperatures are shown in FIG. 3.

In a third experiment, a beta"-alumina tube having a 25.5 mm outerdiameter, and a 23.2 mm inner diameter was uniformly coated on itsexterior with zirconium nitride along a length of 15.7 cm to produce anelectrode 11 microns thick. This tube was provided with a currentcollector and operated in a SHE cell that, when loaded, produced thevoltage-current curves and maximum power versus temperature curves shownin FIG. 4.

Additional advantages and modifications will readily occur to thoseskilled in the art. The invention in its broader terms is, therefore,not limited to the specific details, representative steps, and examplesshown and described. Accordingly, departures may be made from suchdetails without departing from the spirit or scope of the applicant'sgeneral inventive concept.

What is claimed is:
 1. A method of forming an oxide film electrode on asurface of a solid electrolyte substrate, comprising the stepsof:suspending a metallic powder in a liquid having a binder dissolvedtherein; forming a coating of the liquid suspension on the surface ofthe electrolyte, the liquid and binder being evaporable withoutsubstantial carbonization as it is heated past a first elevatedtemperature; heating the coating in a hydrogen atmosphere to a selectedsecond elevated temperature in a range from 800° C. to 1100° C. andhigher than the first elevated temperature for converting the coating toa bonded film and for increasing bondability of the film to theelectrolyte without the electrolyte losing ion conducting properties;subjecting the film in the hydrogen atmosphere to the selected secondelevated temperature for a first time period to effect a required degreeof bonding of the film to the electrolyte; and subjecting the film atthe expiration of the first time period to an oxygen atmosphere for asecond time period effective to convert the bonded film to an oxidefilm.
 2. The method of claim 1, wherein the metallic powder used in, thesuspending step is a metal hydride.
 3. The method of claim 1, whereinthe metal hydride used in the suspending step is selected from a groupconsisting of TiH₂, ZrH₂, and HfH₂, and VH.
 4. The method of claim 1,wherein the liquid used in the suspending step is toluene.
 5. The methodof claim 1, wherein the binder used in the suspending step is amethacrylate.
 6. The method of claim 1, wherein the first elevatedtemperature is in a range from 100° C. to 400° C.
 7. The method 1,wherein the second elevated temperature is 900° C.
 8. The method ofclaim 1, wherein the step of heating the coating in a hydrogenatmosphere includes flowing a stream of dry hydrogen over the coating.9. The method of claim 1, wherein the bonded film formed during the stepof heating the coating in a hydrogen atmosphere is a metal film formedby decomposition of the coating in the hydrogen atmosphere.
 10. Themethod of claim 1, wherein the step of subjecting the film to an oxygenatmosphere includes an atmosphere of nitrogen with oxygen impurities.11. The method of claim 1, wherein the step of subjecting the film to anoxygen atmosphere includes flowing a stream of oxygen over the film. 12.The method of claim 1, wherein the first time period is at least thirtyminutes.
 13. The method of claim 1, wherein the second time period is atleast thirty minutes.
 14. The method of claim 1, wherein the solidelectrolyte substrate is beta"-alumina.
 15. The method of claim 1,wherein the metallic powder used in the suspending step is a metalcompound.